Aliphatic polycarbonates are biodegradable eco-friendly polymers. Poly(ethylene carbonate) and poly(propylene carbonate) as aliphatic polycarbonates can be prepared via alternating copolymerization of carbon dioxide as a monomer with the corresponding epoxide. The use of carbon dioxide is of great environmental value (Reaction 1). A catalyst with ultrahigh activity for carbon dioxide/epoxide copolymerization reaction was developed by the present inventors and is currently ready for commercialization under the trademark Green Pol (Korean Patent No. 10-0853358). The number of carbon atoms in the carbonate linking groups of aliphatic polycarbonates prepared via dioxide/epoxide copolymerization is limited to 2. Poly(ethylene carbonate) and poly(propylene carbonate) as representative aliphatic polycarbonates have limited physical properties, such as low glass transition temperatures of 40° C. and 20° C., respectively, and lack of crystallinity.

Aliphatic polycarbonates whose carbonate linkers each has three or more carbon atoms can be prepared via ring-opening polymerization of the corresponding cyclic compounds (Reaction 2). Such ring-opening polymerization has the advantages that no by-products are formed and final polymers have high molecular weights (e.g., weight average molecular weights of several hundreds of thousands (Pego A P, Grijpma D W and Feijen J, Polymer 2003, 44, 6495-6504); Yamamoto Y, Kaihara S, Toshima K and Matsumura S, Macromol. Biosci. 2009, 9, 968-978). However, the monomeric cyclic compounds are not easy to produce and their use is thus not suitable for the commercialization of aliphatic polycarbonates. That is, the trimethylene carbonate shown in Reaction 2 is currently sold at a price of about 158,000 won per 50 g by Aldrich and is thus unsuitable for use as a monomer for the preparation of general-purpose polymers. The (tetramethylene carbonate) dimer and (hexamethylene carbonate) dimer are not distributed in the market and are produced through complicated isolation and purification processes using enzymes. Accordingly, the use of the dimers is inappropriate for mass production of aliphatic polycarbonates on a commercial scale.
The most appropriate method for mass production of aliphatic polycarbonates whose carbonate linkers each has three or more carbon atoms is associated with the condensation of dimethyl carbonate or diethyl carbonate and various diols (Reaction 3). Dimethyl carbonate and diethyl carbonate are inexpensive compounds that have been produced from phosgene. Efforts have been made to develop processes for the production of dimethyl carbonate and diethyl carbonate using carbon monoxide or carbon dioxide instead of toxic phosgene. The use of environmentally friendly carbon dioxide is more advantageous. Dimethyl carbonate and diethyl carbonate produced by these processes are in practical use at present.
There are many reports in the literature on the preparation of aliphatic polycarbonates via the condensation reaction shown in Reaction 3. However, Reaction 3 for the preparation of aliphatic polycarbonates is slow and has a limitation in increasing the molecular weight of the final polymers. No prior art process is disclosed for preparing high molecular weight aliphatic polycarbonates in an easy manner. Oligomeric macrodiols having —OH groups at both terminals are currently produced and used for polyurethane production. It was reported that macrodiols having a molecular weight as low as ≦2,000 can be produced by condensation of 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol with dimethyl carbonate (DMC) using a calcium catalyst (J. Appl. Polym. Sci. 2009, 111, 217-227). However, the overall reaction time is as long as 36 hours. According to a recent report, macrodiols having a low molecular weight on the order of 1,000 can be produced through a condensation reaction between 1,6-hexanediol and DMC using calcined MgAl hydrotalcites as solid bases (Ind. Eng. Chem. Res. 2008, 47, 2140-2145). In this case as well, the reaction time (≧12 hours) is long. Other reports are also found in the literature on the synthesis of macrodiols with a molecular weight of several thousands and the production of polyurethane using the macrodiols (U.S. Patent Publication No. 2010/0292497; EP 302712; EP 1874846). The synthesis of the macrodiols usually requires a long reaction time of at least 10 hours.
Efforts have also been made to prepare high molecular weight aliphatic polycarbonates. Sivaram et al. reported the preparation of aliphatic polycarbonates having a weight average molecular weight of 6,000 to 25,000 by condensation of DMC with various diols (e.g., 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, and 1,4-bis(hydroxymethyl)cyclohexane) using 1,3-diphenoxytetra-n-butyldistannoxane as a catalyst (Polymer 1995, 36, 4851-4854). The overall reaction time is 11 hours and the reaction temperature is raised to 220° C. The reaction is carried out via a two-step process to increase the molecular weight of the polymers. After step 1, each reaction product is dissolved in methylene chloride and washed with water to remove unreacted diol compound. That is, as a strategy to increase the molecular weights of the polymers, oligomers end-capped with methyl carbonate having a low solubility in water are subjected to a condensation reaction while removing DMC in step 2. U.S. Pat. No. 5,171,830 discloses a process for the preparation of aliphatic polycarbonates including condensing DMC with various diols using a tertiary amine or alkylammonium salt as a catalyst. According to a representative example of this patent, 1,4-butanediol is reacted with excess DMC at 150° C. for 8 hours to prepare mono- or bis(methyl carbonate)esters of 1,4-butanediol and a condensation reaction of the mono- or bis(methyl carbonate)esters is induced while removing volatiles under vacuum or reduced pressure at an elevated temperature up to 200° C. to increase the molecular weight of the polymer. However, the molecular weights of the polymers prepared by this process are only on the order of 2,400 and the end groups of the polymers are capped with methyl carbonate. Recently, Chuncheng Li et al. reported that polymers with a weight average molecular weight of a maximum of 170,000 can be obtained by condensation of DMC and 1,4-butanediol using a TiO2/SiO2/poly(vinyl pyrrolidone) mixture as a solid catalyst (Polym. Int. 2011, 60, 1060-1067; J. Macrom. Sci. Part A: Pure Appl. Chem. 2011, 48, 583-594). The overall reaction time is about 10 hours. They took a strategy to increase the molecular weight of the polymers by preparing oligomers end-capped with methyl carbonate in step 1 and inducing a condensation reaction of the oligomers while removing DMC in step 2. The creation of vacuum or reduced pressure at a high temperature of 200° C. is absolutely required to increase the molecular weights of the polymers. However, under these temperature and pressure conditions, tetrahydrofuran (THF) is formed as a by-product. A high reaction temperature of 200° C. is absolutely important in increasing the molecular weights of the polymers. When the condensation reaction temperature is 190° C. at which no THF by-products are formed, the weight average molecular weights of the polymers are as low as 60,000.
In EP 1134248, pointing to the fact that there is a limitation in preparing high molecular weight aliphatic polycarbonates by condensation of DMC and diols, an attempt to use aliphatic polycarbonate-diols with a molecular weight on the order of 1,000, which have been used for polyurethane production, was made to prepare polymers with a higher molecular weight. Specifically, the aliphatic polycarbonate-diols are condensed with diphenol carbonate (DPC) while removing phenol to increase the molecular weight of the final polymers. Despite this attempt, the molecular weight of the polymers is only on the order of 3,000. DE 1031512 reported a process for preparing poly(hexamethylene carbonate) with a molecular weight on the order of 25,000 by condensation of a low molecular weight aliphatic polycarbonate-diol and phenyl chloroformate at 250° C. However, the use of toxic expensive materials such as phenyl chloroformate makes the process less attractive. Further, the reaction temperature is too high, posing a danger that a considerable amount of THF may be formed from 1,4-butanediol.
Some reports have been published on the synthesis of aliphatic polycarbonates using more reactive diphenol carbonate (DPC) instead of less reactive DMC (U.S. Pat. No. 6,767,986; EP 2033981; EP 2036937). However, taking into consideration that DPC is produced with a low conversion rate from DMC, the use of DPC rather than DMC would not be preferable in condensation reactions for the preparation of aliphatic polycarbonates.